The present invention relates to an ultrasonic fluid flow measuring method and apparatus including a sigma-delta bandpass analog-to-digital converter.
An ultrasonic fluid meter comprises two ultrasound transducers defining a measurement path between them. Each transducer is used alternately in emission mode and in reception mode.
The principle of measuring a fluid rate acoustically consists in determining the speed of the flowing fluid, by determining the time the acoustic signal takes to propagate between the two transducers both in the upstream direction and in the downstream direction relative to the fluid flow. The travel time of the ultrasound wave is calculated on the basis of measuring time and/or measuring phase.
The flow rate and the volume of fluid that has flowed over a given length of time can then easily be determined on the basis of the measured fluid speed.
Such ultrasound apparatus is well known to the person skilled in the art and is described, for example, in European patent application EP 0 807 824. The same applies to the acoustic measurement method as described for example in European patent application EP 0 852 725.
Fluid meters relying on the ultrasound principle to measure flow rate are entirely self-contained and they do not depend on an electricity distribution network. These meters contain electronics that is becoming ever more sophisticated, and they make it possible to improve measurement performance, giving consumers various kinds of information about consumption and enabling consumption to be read remotely and/or bills to be paid remotely, and they are powered by means of a battery of limited lifetime, which lifetime depends very strongly on the architecture of the electronic circuits used.
The architecture of a complete system for acquiring and processing measurements in a prior art ultrasound fluid meter is shown in FIG. 1. Such an ultrasound fluid meter comprises two transducers 1 and 2 placed in a cavity 3 along which a fluid flows. The transducers are both connected to a switch unit 4 in such a manner that when the first transducer 1 operates in emission mode the second transducer 2 operates in reception mode, and vice versa. When the first transducer 1 emits an ultrasound wave UW which propagates in the fluid flow, the transducer 2 receives said ultrasound wave after a length of time that is characteristic of the flow speed, and it transforms the ultrasound wave into an analog signal. The switch unit 4 is connected to an amplifier 6 having programmable gain which serves to provide full-scale amplification and filtering of the analog signal for application to an analog-to-digital converter 8. The programmable gain amplifier 6 is connected to an 8-bit analog-to-digital converter operating at a sampling frequency of 320 kHz. The analog-to-digital converter 8 delivers a digital signal as is required for determining the propagation time of the acoustic signal UW between the two transducers 1 and 2. The analog-to-digital converter 8 is connected to a random access memory (RAM) 10 having capacity of 2xc3x97256 bytes which stores the signals, until they can be processed by the microcontroller 12. The microcontroller 12 which processes stored signals and calculates results relating to flow rate is connected to a set of various units 13 serving, for example, for the purposes of display, communication with the outside, managing energy saving modes, and storing operating data. The microcontroller 4 is also connected to a sequencer 14. The sequencer 14 controls the sequences of ultrasound waves fired by the transducers 1 and 2 via a transmission buffer 16 comprising a digital-to-analog converter and an amplifier, and it also controls the sampling performed by the analog-to-digital converter 8 and the storing of signals in the memory 10. A battery (not shown) operates in conventional manner via a set of connections (not shown) to provide the energy necessary to enable the various components to operate.
The combination of a programmable gain amplifier and an analog-to-digital converter corresponds to architecture which is complex and which consumes 30% to 40% of the energy requirements of the electronics of the meter. In addition, such an analog-to-digital converter introduces quantization noise while digitizing, thereby degrading measurement accuracy. Such a xe2x80x9cconventionalxe2x80x9d analog-to-digital converter converts a signal with constant resolution providing its frequency lies in the range DC to half the sampling frequency.
It is known to the person skilled in the art that transformation of an analog signal into a digital signal by means of an analog-to-digital converter is a major source of error commonly known as quantization noise. A technique known to the person skilled in the art for reducing such quantization noise is using sigma-delta conversion (see for example xe2x80x9cDelta-sigma data convertersxe2x80x94theory, design, and simulationxe2x80x9d by Steven R. Norsworthy et al., IEEE Press, New York, 1997). Noise reduction is obtained by sigma-delta conversion because the architecture of a sigma-delta converter enables it to take account of the conversion errors made during past conversions in order to correct future conversions.
Furthermore, another aspect of sigma-delta conversion relates to the particular way the information that results therefrom is encoded. Sigma-delta conversion is a principle for encoding information on a small number of bits, sampled at a high frequency so as to enable resolution to be increased subsequently. This conversion principle is based on operation that is analogous to that of delta conversion, which consists in encoding the difference between the amplitude of a sample and the amplitude of the preceding sample. For example, when encoding on a single bit, a sigma-delta converter generates a binary output stream (alternating xe2x80x9c0sxe2x80x9d and xe2x80x9c1sxe2x80x9d) constituted by a periodic regime whose fundamental period is proportional to the input voltage. The converter responds as a voltage-to-frequency converter which is synchronized on a sampling clock. A xe2x80x9cdecimatorxe2x80x9d digital filter is placed at the output from the sigma-delta converter and converts the signal encoded on a small number of bits at high frequency into a signal at a lower bit rate but encoded on a larger number of bits.
The principle of sigma-delta conversion can be extended to converting signals centered around a particular frequency. The converter used is then a bandpass sigma-delta converter. The filter of the converter which was previously an integrator is replaced by a resonator. The digital filter at the outlet from the sigma-delta converter is no longer a decimator but a bandpass filter followed by a demodulator. In the field of telecommunications, and in particular in the field of digital radio, it is known to use bandpass sigma-delta analog-to-digital converters in order to eliminate quantization noise (see for example xe2x80x9cA fourth-order bandpass sigma-delta modulatorxe2x80x9d by Steven A. Jantzi et al., IEEE Journal of Solid State Circuits, Vol. 28, No. 3, March 1993, pp. 282 to 291).
The object of the present invention is to remedy the drawbacks of the measurement acquisition and processing system of prior art ultrasound fluid meters, and in particular to reduce the complexity and the power consumption of the digitizer.
Another object of the present invention is specifically to reduce quantization noise during digitization of the analog signal and to increase the performance of the converter.
In the invention, these objects are achieved by replacing the prior art digitizing system by a sigma-delta converter.
More precisely, the present invention provides ultrasound apparatus for measuring a fluid flow rate, the apparatus comprising:
first and second transducers placed in the fluid whose flow rate is to be determined, one of the transducers, also referred to as the xe2x80x9cemitterxe2x80x9d transducer, operating in emission mode while the other transducer, also referred to as the xe2x80x9creceiverxe2x80x9d transducer, operates in reception mode, the emitter transducer being designed to emit an ultrasound wave into the fluid and the receiver transducer being designed to transform said ultrasound signal into an analog signal; and
processor means for processing said analog signal, the processor means being connected to the receiver transducer and being designed to transform said analog signal into a digitized signal used for determining the fluid flow rate;
the apparatus being characterized in that said analog signal processing means comprise a bandpass sigma-delta converter comprising:
a bandpass loop filter whose input is connected to the output of said receiver transducer;
an analog-to-digital converter whose input is connected to the output of said loop filter, the output of said analog-to-digital converter forming the digital output of said analog-to-digital converter; and
a digital-to-analog converter forming a feedback loop, connecting the output of the analog-to-digital converter to the input of said loop filter.
The transducers used in apparatus of the invention are piezoelectric type transducers possessing a bandpass transfer function that is limited in frequency, e.g. 40 kHzxc2x11.5 kHz. Since the useful information lies solely in this frequency band, it is advantageous to amplify and convert only those signals which lie in this frequency band.
In a preferred second embodiment of the present invention, the ultrasound apparatus for measuring fluid flow rate comprises a bandpass sigma-delta converter having a bandpass loop filter that is constituted by the receiver transducer.
The transducer thus acts in alternation as a receiver and as a bandpass filter in the loop of the bandpass sigma-delta converter, thus making it possible to optimize analog-to-digital conversion in the frequency band of interest.
The present invention also provides an ultrasound method of measuring a fluid flow rate that includes a bandpass sigma-delta analog-to-digital converter.
In the invention, an ultrasound method of measuring a fluid flow rate between two transducers, in which the fluid flow rate is determined by measuring the propagation time and/or by measuring the phase shift of acoustic signals propagating in the flowing fluid between two transducers in the upstream and downstream directions of the fluid flow, comprises:
an emission step consisting in emitting an acoustic signal UW into the fluid whose flow rate is to be determined;
an acoustic-to-analog conversion step consisting in transforming said acoustic signal UW into an analog signal S2;
an analog-to-digital conversion step of order N consisting in transforming said analog signal S2 into a digital signal S3; and
a step of determining acoustic phase shifts and propagation times consisting in determining the acoustic phase shifts and the propagation times on the basis of the digitized signal S3;
the method is characterized in that the analog-to-digital conversion step of order N implemented by a sigma-delta converter comprises:
an estimation step consisting in estimating the quantization error qNxe2x88x921 that occurs during the digitizing step of order Nxe2x88x921, for use in the digitizing step of order N;
a subtraction step consisting in subtracting the estimated quantization error qNxe2x88x921from the analog signal S2; and
a digitizing step consisting in digitizing the analog signal S2 minus the estimated quantization error qNxe2x88x921.
Advantageously, the acoustic-to-analog conversion step consisting in transforming said acoustic signal UW into an analog signal S2 comprises:
a step of converting the acoustic signal UW into analog signal S1; and
a step of truncating the analog signal S1 into an analog signal S2.
In advantageous manner, the step of determining the acoustic phase shifts and the propagation times consisting in determining the acoustic phase shifts and the propagation times on the basis of the digitized signal S3 comprises:
a step of filtering the digital signal S3 into a filtered digital signal S4; and
a step of calculating acoustic phase shifts and/or propagation times on the basis of the filtered digital signal S4.
Advantageously, the estimation step consisting in determining an estimate of the quantization error is implemented by the receiver transducer.
Other characteristics and advantages of the invention appear in the following detailed and non-limiting description of various embodiments given with reference to the accompanying figures, in which:
FIG. 1 is a block diagram of the complete measurement acquisition and processing system of a prior art ultrasound fluid meter;
FIG. 2 is a block diagram of the acquisition system of an ultrasound fluid meter of the invention;
FIG. 2.a is a time plot of the signal S1;
FIG. 2.b is a spectrum plot of the signal S1;
FIG. 2.c is a time plot of the signal S2;
FIG. 2.d is a spectrum plot of the signal S2;
FIG. 2.e is a time plot of the signal S3;
FIG. 2.f is a spectrum plot of the signal S3;
FIG. 2.g is a time plot of the signal S4;
FIG. 2.h is a spectrum plot of the signal S4;
FIG. 3 is a block diagram of the acquisition system of the ultrasound apparatus for measuring fluid flow in the preferred embodiment of the invention;
FIG. 4 is a timing diagram of signals for controlling the switches while the transducer 1 is emitting and the transducer 2 is receiving in the apparatus of FIG. 3;
FIG. 5 is a timing diagram of signals for controlling the switches while the transducer 1 is receiving and the transducer 2 is emitting in the apparatus of FIG. 3;
FIG. 6 shows the various steps of the method of the invention; and
FIG. 7 shows a variant embodiment of the FIG. 3 apparatus.